1. Field of the Invention
The present invention relates to an electrostatic capacitive pressure sensor and more particularly to an electrostatic capacitive pressure sensor employing silicon diaphragms.
2. Description of the Related Art
FIG. 1 is a sectional view of a conventional electrostatic capacitive pressure sensor.
As shown in FIG. 1, a thin diaphragm portion 1 is prepared by supersonic machining or by etching the central portion of one side of a silicon flat plate. A metal pad 3 (e.g., of Al, Au or Mo) is formed on the diaphragm 1 from which a lead wire 13 extends. A borosilicate glass base 2 whose coefficient of thermal expansion is close to that of silicon (3.2.times.10.sup.-6 /C..degree.) is provided with a through-hole electrode 4 made of Cr-Au, Cr-Ni-Au or the like and is bonded to the thick portion of the silicon diaphragm 1 through a known electrostatic bonding method, so that a measuring chamber 15 is formed. A metal pipe 7 of Kovar (produced by Westinghouse Electric Corp.) or Fe-Ni may be bonded onto the borosilicate glass base 2 with a junction layer 6 whose coefficient of thermal expansion is as low as 5.times.10.sup.-6 /C..degree. to form a port for introducing pressure P.
The above-described conventional sensor has many disadvantages and drawbacks. Although the reproducibility of the measuring gap between the silicon diaphragm 1 and the electrode 4 is superior according to the electrostatic bonding method, there is a loss of airtightness because of foam left in the junction (the portions that have not been coupled).
There is an alternate method to the electrostatic coupling method of bonding the silicon diaphragm 1 and the glass base 2. In this alternate method, glass is applied to the electrostatic junction 8 of FIG. 1, the glass having a low coefficient of thermal expansion. However, the gap between the diaphragm and the electrode is difficult to reproduce with high dimensional accuracy, although airtightness is excellent. Particularly when the pressure sensor is intended for miniaturization, the gap should be as small as 10 .mu.m or less and it is almost impossible to provide a gap as small as 10 .mu.m or less with the preferred reproducibility. Although a thin film of Cr-Ni-Au may be deposited and formed on the borosilicate glass 2 through evaporation by making use of the Au-Si eutectic reaction, the measuring gap is also difficult to accurately reproduce.
Because a pair of opposite electrodes are used for measurement, the dielectric constant of the pressure medium within the measuring chamber is affected by temperature. Consequently, the resulting temperature characteristics of the sensor are bad, which makes it impossible to obtain a signal properly proportional to the pressure.
An object of the present invention is to provide a highly accurate electrostatic capacitive pressure sensor in terms of temperature characteristics, and more particularly to provide an electrostatic capacitive pressure sensor having a gap between a silicon diaphragm and an electrode which can be reproduced with high accuracy and efficiency. A further object of the present invention is to airtightly bond the pressure introducing chamber.